Let's start off with the weight - it's way heavy. It's not until you spend a few hours walking around with it do you really notice it. Picking it up for a couple of minutes is not a great test of the strain it will put your arms under.
I took my kit (10-22mm, 24-105 IS, 50mm 1.8, 100-400mm IS, 1.4x) to the zoo yesterday and this was the only lens that I needed.
I never needed to go wider than 100mm, and at 400mm I got head shots of the lions at around 50 metres. The results were very sharp and the bokeh was simply amazing (all handheld, no tripod)
I've read a lot of negatives about the push-pull zoom, but I can't recommend it highly enough. It responds quickly and is a pleasure to use.
I didn't use the 1.4x extender at the zoo, but in other outings it has proved to produce great results when I need that extra length (560mm on a 1.6x crop body is way cool).
The IS works well and really is a must when shooting over 100mm.
I had been considering the 70-200mm F/4 IS over this lens, and even though this was AU$600 more, I think it was well worth it.